This invention relates to sputtering devices and methods including magnetic enhancement of such devices.
FIGS. 1 and 2 are cross-sectional and perspective views respectively of a representative prior art planar magnetron sputtering device comprising inner magnet 10 and outer magnet 12 (both of which usually comprise a number of sections) where the magnets are shunted by an iron pole plate 14. Disposed above the magnetic structure is a cathode or target 16 (not shown in FIG. 2). The magnetic lines of force are as shown in FIG. 1 where they exit from and return through cathode 16, a similar technique being employed in U.S. Pat. No. 3,878,085 where the magnetic lines also enter and exit from the cathode surface.
An electric field is established between (a) a ring-like anode 17, which may be disposed around and spaced from cathode 16, (or the chamber wall may serve this function) and (b) the cathode or target whereby electrons are removed therefrom. Due to the configuration of the lines of magnetic force (the illustration of which is approximate), the removed electrons tend to concentrate in regions A where the lines of force are substantially parallel to the upper surface of target 16. There the electrons ionize gas particles which are then accelerated to the target to dislodge atoms of the target material. The dislodged target material then typically deposits as a coating film on an object to be coated. Assuming the object to be coated is in strip form or is mounted on a strip moving in the direction of the arrow shown in FIG. 2, the object will be uniformly coated, the strip being narrower in width than the length of the sputtering device.
Once the ionizing electrons are removed from the target, they travel long paths because they circulate in a closed loop defined between inner magnet 10 and outer magnet 12, the loop being above target 16. Hence, the electrons are effective in ionizing the gas particles. However, since most of the ionizing electrons are concentrated in regions A, the ionized gas particles will mainly erode cathode 16 in regions A'. Such uneven disintegration of the target is undesirable in that the target materials are most often extremely pure and accordingly, very expensive.
Another prior art arrangement is shown in cross-section in FIG. 3 where parallel magnets 18 and 20 are employed with pole pieces 22 and 24. However, this configuration is essentially the same as that of FIGS. 1 and 2 in its function and is subject to the same shortcomings.
At FIGS. 5 and 9 of aforementioned Pat. No. 4,162,954, there are disclosed magnetic field sources 28 which render the lines of force more parallel to the surface of target 16, these Figures corresponding to FIGS. 4 and 5 of the present application. The source 28 comprises a plurality of flexible magnetic tapes which may be concentrically arranged or stacked to form a flat coil as shown in FIG. 4. Each loop of the coil may comprise a strip of tape where the ends of each strip abut one another. Together the strips are substantially equivalent to a solid magnet where the directions of the flux in each strip magnet are represented by arrows in FIG. 4 and where the north and south poles of this "solid" magnet are as shown, it being understood that the polarities shown are illustrative and may be reversed, if desired. Rather than employing concentric or stacked strips as shown in FIG. 4, a single strip can be tightly wound to provide a spiral configuration which is also very effective. Typically the strips of flexible magnetic tape are oriented ferrite impregnated rubber strips 1/2 inch wide and 1/16 inch or 1/8 inch thick (such as PL-1.4H made by Minnesota Mining and Manufacturing Co.). Further, rather than tapes, ferrite block magnets (typically 1/4.times.1 inch thick ferrite magnets made by Arnold Magnetics, Inc. or Crucible Iron and Steel Co.) may also be employed to construct a configuration corresponding to that of FIG. 4.
In spite of the improvements effected by the FIG. 4 embodiment, it still suffers from some uneven target utilization. Where the lines of flux enter the center line of the target at about 45.degree. or more, there is no erosion of the target. The lack of erosion in the target center is of particular concern due to the great cost of most of the targets. Increasing the area significantly eroded before any point erodes all the way through the target is thus of great importance. Accordingly, the magnetic field source 28 of FIG. 5 may be employed whereby the magnets are tipped away from the perpendicular orientation shown in FIG. 4 to further enhance field parallelism. The angle of the magnets with respect to the perpendicular can fall within the 40.degree.-60.degree. range shown in FIG. 5 and preferably this angle should be 50.degree.-55.degree.. Special orientations of the magnets to change the pattern of erosion are readily implemented when the flexible magnets are used. Since the field is more parallel to the target surface in the embodiments of FIGS. 4 and 5, uniformity of target erosion is accordingly enhanced compared to the prior art embodiments of FIGS. 1-3. The prior art embodiments such as that of FIG. 1, however, are highly flux conservative in that no flux penetrates plate 14. That is, practically all of the flux is disposed above the magnetic field source. Hence, practically the entire flux of center magnet 10 can be used and about 50% of that from outer ring 12. In the embodiments of FIGS. 4 and 5, no particular emphasis is given to flux conservation because there is sufficient flux for most applications and to conserve flux without specific reason is not cost effective. However, in the embodiment of FIG. 5, where the magnets 28 are tipped toward the center of the structure to further narrow the loss of target material in the center, the flux levels projected upward tend to be somewhat marginal. That is, the criteria previously established for the erosion near the center of the target were that of a parallel field at approximately 3/8 inch of about 80 Gauss, and a resultant field angle of 45.degree. or less. When the magnets are tipped to achieve the best resultant field angles, the parallel field level is difficult to achieve. The reason for this is as follows.
Referring to FIG. 6A, there is shown a single magnet 28' which may correspond to a single ferrite bar magnet or a plurality of ferrite strips of the type described before with respect to FIG. 4. Magnet 28' may also correspond to the right half of the FIG. 4 configuration. The direction of magnetic field projection is .alpha.=0. As .alpha. becomes larger, the projection power decreases. When two magnets 28' and 28" are brought toward each other in opposition as in FIG. 6B, the fields butt against each other. If the magnets are tipped partially downward as in FIG. 5, the influence of one field upon the other is to force a large proportion of the flux downward and away from target 16. In FIG. 6A, it can be seen that the proportions of flux returning to the south end of the magnet are the same above and below the magnet. When butted head on as in FIG. 6B, the flux is forced to exit from the top and bottoms of the magnets, but the proportion above and below remain essentially equal. If they are forced together only at the upper edge of their faces as in FIG. 5, the proportion of flux returning below will become significantly greater and thus unavailable for sputtering of target 16. However, as .alpha. approaches 55.degree. from horizontal (note .alpha. also corresponds to the angle shown in FIG. 5), the upward projected part of the flux seems to come from a very narrow region and thus, erosion of the center of the target tends to be optimized assuming the parallel field strength is sufficient. That is, the region C in FIG. 6B becomes very small.
Assuming magnets 28' and 28" are assembled as stacks of flexible magnet strips, the flux levels become very large. As the magnet stacks are moved together, as in FIG. 6B, the flux pressure builds to extreme levels, and the flux is deflected out the top and bottom of the stacks at the region C. Each element of the magnet stacks 28' and 28" restricts the reverse flow of flux in the .alpha.=0 direction. As can be seen in FIG. 6B, the resultant flux flow is nearly perpendicular to .alpha.=0. Thus, the magnets of FIG. 6B are not oriented at the best angle with respect to each other to lessen this flux leakage out of the relatively wide region C. Angling the magnets downward about 55.degree. as in FIG. 5 gives the best reduction of this leakage without overly exposing the backs of the magnets to destroy the required polarity for causing the flux. Thus, by this expedient the region C is reduced to a very narrow flux flow channel in the center of the structure; but most of the flux is lost out the bottom in the process.
Accordingly, it is an object of this invention to provide an improved magnetically enhanced sputtering device and method where the flux in a magnetic field source beneath the target may be angled with respect to the target and yet the strength of the parallel field over the central portion of the target is sufficient to effect sputtering thereof.
Even in the more simple structures, such as that of FIG. 4, without tipped magnets, there is a loss of nearly 50% of the flux out the bottom of the magnet. In these structures, targets can be placed on both sides of the magnet for simultaneous sputtering thereof. When an outer frame 30 is added to the structure of FIG. 4, as in FIG. 7 (which corresponds to FIG. 30 of aforementioned Application Ser. No. 19,284), the outer end of magnet 28 is effectively tipped upward while the center thereof is also effectively upwardly tipped by inner magnet 32.
As shown in FIG. 7, there is magnetic coupling at the bottom of the structure between magnets 28 and 30 and rejection at the top. This significantly lengthens the flux path from the bottom center of the magnet structure relative to that from the top center. This expedient thus reduces the magnitude of the bottom flux flow as does the magnet 32. As discussed in Application No. 19,284, magnet 32 thus enables the sputtering of thicker targets (greater than 1/2 inch and typically 1 inch thick for heavier industrial coating applications such as glass, auto parts, plastic films, etc.) although at the expense of minimal sputtering of the target center due to the perpendicular orientation of the flux in magnet 32.
Accordingly, it is another object of this invention to provide an improved magnetically enhanced sputtering device and method wherein the percentage target utilization of thick targets is significantly improved.
Sputter cathodes such as those of FIGS. 1 and 4 have tended to be singly targeted. There are, however, cases in which multiple targets are desirable, provided in most situations that only one is operating at a given time.
Accordingly, it is a further object of this invention to provide an improved sputter cathode wherein a plurality of different target materials may be sputtered in a predetermined sequence.
Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawing.